We have characterized the Na+/H+ exchanger (NHE) isoforms expressed in rat renal cortical tubule fragments. Amiloride sensitivity of the Na+-dependent intracellular pH (pHj) recovery in suspended tubules that had been acid loaded by an NH4+ prepulse was determined in nominally CO2/HCO3--free solution, using the fluorescent pH-sensitive dye 2′, 7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In the presence of 140 mM extracellular Na+, 800 μM amiloride inhibited the rate of Na+-dependent pHi recovery by only 65%, demonstrating the presence of a Na+-dependent amiloride-insensitive H+ extrusion system. This system was not affected by 4-acetamido-2′-isothiocyanostilbene-2, 2′disulfonic acid but was activated by 4, 2′-diisothiocyanostilbene-2, 2′-disulfonic acid. Lowering extracellular Na+ concentration permitted 300 fiM amiloride to completely inhibit Na+-dependent pH; recovery. These results can be explained by the expression of a Na+/H+ exchange with the pharmacological properties of NHE4. Using reverse transcriptasepolymerase chain reaction, we found specific mRNA for NHE1, NHE2, NHE3, and NHE4 isoforms in the renal cortex. Immunohistochemical studies using polyclonal antibodies against rat NHE4 peptide demonstrated that NHE4 is heterogeneously expressed on basolateral membrane domains of cortical tubules. These results strongly suggest that amilorideinsensitive Na+/H+ exchange expressed in renal cortical tubule suspensions is mediated by NHE4. anti-Na+/H+ exchanger 4 peptide. Copyright ©1997 the American Physiological Society.